The present invention relates to a method and apparatus for recording digitized data on a magnetic recording medium and reproducing the recorded data and, more particularly, to a method and apparatus for digital magnetic recording and reproduction of data which is applicable to signals in the audible band which are modulated by the NRZI (Non-Return-to-Zero Inverted) system.
Methods and apparatuses heretofore known for high-density digital data recording and reproduction generally employ a modulation system which is capable of causing self-clocking. When digital data is to be read from a magnetic recording medium, the timing of a magnetic flux reversal or transition is detected in response to a signal induced in a magnetic head upon the flux reversal. The phase of a data read clock is determined on the basis of the detected flux reversal timing, which in turn determines a data read timing. Thus, self-clocking causes a reproduced signal to determine a reproduction clock timing for itself.
The ease of self-clocking becomes greater as the flux reversal is caused more frequently by digital data recorded on a recording medium. As well known in the art, for easier self-clocking, it is desirable that the maximum flux reversal interval be limited to a certain value to allow the reversal to surely exist, and that the minimum flux reversal interval be also limited to a certain value to minimize interference between waveforms. Various digital modulation systems have been proposed to satisfy such demands.
Typical examples of digital modulation systems hitherto known may be the RZ (Return-to-Zero) system, NRZ (Non-Return-to-Zero) system, NRZI (Non-Return-to-Zero Inverted) system and PE (Phase Encoding) system. Of these systems, the NRZ or NRZI system is most fundamental and has the following features:
A modulator is needless or requires only a simple construction. PA0 The minimum flux reversal interval can be made long and the required transmission band, narrow. PA0 The system is suitable for high-density recording. PA0 Self-clocking is generally difficult to occur because the maximum flux reversal interval is infinite. PA0 Particularly, the NRZI system is desirably applicable to differentiation type recording media such as magnetic recording media, because information is contained in each flux reversal.
Now, signals in the audible band as typified by voice and music signals are free from d.c. components. In this respect, self-clocking will not always be impossible when the NRZI modulation system is used to record digital data on a magnetic recording medium for recording and reproducing such audible band signals. This opens the way for high-density data recording and reproduction by use of an apparatus having a simple construction. Still, the audible band signals in nature involve low signal levels which appear very frequently, and even involve no-signal periods. In this regard, sufficient self-clocking cannot be expected from the NRZI modulation system.
The inconvenience discussed above may be eliminated by any one of conventional implements such as employing a conversion rule to cause flux reversal during demodulation or adding an M-series code to give randomness which will insure a certain degree of flux reversal. However, these known implements are not satisfactory because they lack due consideration of the described characteristics of audible band signals.